1. Field of the Invention
The invention relates to a method for forming a titanium thin film, more particularly to a method for forming a titanium thin film by CVD method.
2. Description of the Related Art
Titanium and titanium alloy have been conventionally used for a wiring or contact portion of semiconductive devices. Especially, they play an important role in the contact portion. For example, titanium (Ti) and titanium silicide (TiSi.sub.x) are known as a contact metal material capable of forming an excellent contact because of their low resistance. Titanium nitride (TiN.sub.x) is well known as a barrier metal material having a high barrier. Sputtering method and CVD method are well known as a technique for forming a thin film of such materials. In case of forming a titanium film, sputtering methods are generally used.
In recent years, a diameter of contact portion has been reduced in accordance with minimization of semiconductive devices, and a thickness of interlayer insulating film has been increased for planalization of the device. Accordingly, the shape of the contact hole has been changed to be great in depth and small in diameter, in other words, an aspect ratio of the contact hole has been increased. In such a contact portion, it is necessary to form a titanium film with sufficient thickness at the bottom of the contact hole in order to realize a good contact with titanium. However, good coverage and the film with a sufficient thickness at the bottom of the contact hole was not obtained by the conventional sputtering method for forming a titanium thin film. Further, as seen from FIG. 3, an over-hang 14 of titanium thin film 13 may be formed in a contact hole 12a of SiO.sub.2 film 12 extending to a silicon substrate 1. When titanium thin film 13 was embedded in the contact hole 12a, a void 15 may occur owing to the over-hang 14 as shown in FIG. 4. This is a problem for embedding the titanium film 13 into the contact hole 12a.
Under these circumstance, a method for forming a titanium thin film using CVD methods, which can provide good coverage, has been developed. However, there are several drawbacks of forming a titanium thin film in a contact hole by the use of titanium tetrachloride that is generally known as a material gas for forming a titanium thin film.
In the process for forming a titanium thin film, a substrate needs to be heated. Heating the substrate to a level of a decomposition temperature of titanium tetrachloride (about 900.degree. C. or more) results in too much elevated temperature in the substrate, whereby reaction between Ti and Si can not be controlled. Besides, aluminum alloy as used for wiring may be melted at such heating. Employing plasma energy in place of thermal energy deteriorates the coverage in CVD method. In addition, using titanium tetrachloride as material gas results in a titanium film containing chlorine (Cl). In other words, using titanium tetrachloride has a drawback of allowing wirings to be corroded owing to the difficulty of impeding the infiltration of chlorine content in the titanium film when using aluminum alloy as wirings in the subsequent process.
In this way, the CVD method using titanium tetrachloride has difficulties in producing a titanium thin film with only a small amount of impurities, with good coverage, and at low temperature.
Further, a method using CVD technique with an organic compound as materials has been developed for forming a silicon oxide thin film and titanium nitride film because organic compounds has relatively low decomposition temperature. For example, a method for forming a film of organic compound, which is a solid or liquid state at room temperature, using vapor pressure at a temperature lower than the decomposition of the organic compound is proposed. Japanese Unexamined Patent Publication 1-290771/1989 discloses a method for forming a titanium thin film by thermal decomposition of organic titanium compounds formed of an organic compound having a cyclic structure and titanium connected to a .pi.-electron of the organic compound. The organic titanium compounds used for forming titanium thin film as a material gas rarely have a sufficient vapor pressure at a temperature lower than decomposition. On the other hand, various organic titanium compounds, especially organic compounds having a cyclic structure, is very unstable and difficult to treat. Further, there is a defect that organic compounds are left in the titanium thin film after the decomposition, thereby making it difficult to form a titanium thin film with only a few amount of impurities. Thus, the method for forming a titanium thin film by CVD method has not sufficiently improved.